Massive MIMO systems scale up conventional MIMO systems by possibly orders of magnitude, e.g., to hundreds of antennas at a Base-Station (BS), to simultaneously serve tens of User Equipments (UEs) in the same time-frequency resource. With the capabilities of aggressive spatial multiplexing and large array gains, a massive MIMO system can achieve great capacity increase [1]-[3]. In addition, it could be built with inexpensive low-power components. It also has the potential of reducing the latency of the air interface, simplifying the media access layer, as well as increasing the robustness to both unintentional artificial interference and intended jamming. In general, massive MIMO systems are considered in Time-Division Duplex (TDD) mode, taking advantages of the channel reciprocity between the uplink and downlink. Channel estimation using reciprocity in Frequency-Division Duplex (FDD) massive MIMO is possible by the methods described in our provisional patent application 61/919,032 “Method for Acquiring Channel State Information In FDD MIMO Wireless Networks” filed on Dec. 20, 2013. Moreover, Orthogonal Frequency-Division Multiplexing (OFDM) is still the prevalent technology to multiplex UEs for the whole bandwidth as the 4th Generation (4G) LTE communication systems and is well suited for MIMO systems. Massive MIMO with OFDM could increase spectrum efficiency more than ten times of the conventional systems with relatively simple implementation.
When MU-MIMO is employed in conventional TDD communication systems, e.g., 3GPP LTE/LTE-A, the Sounding Reference Signal (SRS) transmitted by a UE is mainly used by the BS to measure the wireless channel coefficients between itself and the UE. Then, the estimated channel coefficients are used to compute the precoding matrices for downlink data transmission. For the uplink signal detection, the BS has to estimate the channel coefficients between itself and UEs based on the received pilot signals specifically for data demodulation first, e.g., Demodulation Reference Signal (DMRS). Then, it computes the detection matrix on each radio resource unit to separate the signals belonging to each UE from the received signals, in which the signals from multiple UEs are superposed. However, this process is not feasible in a massive MIMO system. The reason is that as the numbers of receiving antennas and multiplexed UEs are increased to hundreds and more than ten respectively, the computation of detection matrices requires huge hardware resources, especially when the system bandwidth is large, e.g., 20 MHz. As a result, it increases cost and causes unacceptable processing delay which cannot meet the typical requirement of Radio Access Network (RAN). Hence, a whole new uplink signal detection process is provided in this patent for massive MIMO systems to ensure that the performance of uplink transmission is no worse than conventional systems, while the process delay is reduced to meet the requirement of RAN. FIG. 1 illustrates a typical MU-MIMO communication system in the uplink, where the BS 1 communicates with three UEs 2, UE1, UE2, and UE3, at the same time with the same frequency resource. Take one specific resource unit as an example, after passing through three different wireless channels 3, h1, h2, and h3, signals s1, s2, and s3 transmitted by the three UEs respectively are superposed at the BS's receiving antennas. Combining the receiver noise n and the neighboring cell interference I, the received signals by the BS can be modeled as y=h1s1+h2s2+h3s3+n+I. FIG. 2 provides the process of signal detection in conventional MU-MIMO communication systems. It begins 4 when the BS estimates the channel vector of each UE in the MU-MIMO group on a specific radio resource for data transmission through the pilots inserted in the data region 5. Then, the BS calculates the detection matrix of the specific radio resource 6. After that, the BS applies the detection matrix to separate the data belonging to each UE in the MU-MIMO group 7 and the process ends 8.
The antennas of massive MIMO systems can be distributed in two ways. The first one is centralized antenna systems, where all antennas are located in one place and it needs large space to fix the huge antenna array if the carrier frequency is relative small, e.g., 2 GHz. The second one is distributed antenna systems, where all antennas are divided into several groups and each group is fixed at a different place. The Radio Frequency (RF) signals of these groups can be passed to the baseband through fibers or other interfaces. FIG. 3 illustrates a centralized antenna system where a BS 1 with a large number of antennas 9 and a centralized baseband processor 10 communicates with multiple UEs 2 simultaneously. FIG. 4 illustrates a distributed antenna system where three remote radio heads 11 with their own antenna arrays 9 respectively is connected to a common baseband processor 10 through fibers 12 and communicate with multiple UEs 2 simultaneously.
This invention presents embodiments that provide the signal transmission and detection methods as well as the relative processes for the downlink and uplink transmission in massive MIMO systems.